The mechanical paradox of low-angle normal faults: Current understanding and open questions

摘要

Low-angle normal faults, LANF, (dip < 30°) have been proposed as key-structures for accommodating crustal extension. In contrast, frictional fault reactivation theory predicts that slip on LANF is extremely unlikely: this prediction is consistent with the absence of moderate-to-large earthquakes on normal faults dipping less than 30°.In order to discuss this discrepancy I will analyse and integrate: 1) geological data from 9 LANF, 2) the dip-range of earthquake-ruptures in extensional environments, and 3) frictional fault mechanics.LANF fault zone structure is represented by two end members: a) a thick mylonitic shear zone superposed by cataclastic processes and some localization; 2) a discrete fault core separating hangingwall and footwall blocks affected by brittle processes. LANF act as preferential channels for fluid flow and in some cases they promoted fluid overpressure. Fluid-rock interactions along some detachments favour the development of phyllosilicates that in general are characterised by low frictional strength, μ < 0.4, and inherently stable, velocity-strengthening frictional behaviour. The low friction coefficient of the phyllosilicates can explain movements on LANF and the velocity strengthening behaviour of the phyllosilicates implies fault creep and therefore can be used to explain the absence of moderate-to-large earthquakes on LANF in seismological records.However in my view, the integration of the three datasets does not provide a simple mechanical solution for the LANF paradox since it leaves two important open questions. First a widespread development of phyllosilicates does not seem to be a common feature for most of the exhumed LANF that on the contrary show the typical fault rocks of the brittle and seismogenic crust. Second, although some brittle detachments reactivated pre-existing ductile shear zones, others formed as gently dipping structures within a brittle crust characterised by a vertical σ_1: a well constrained mechanical explanation for this second class of structures is lacking.